The impact of feed composition on the stability of air classification in waste-to-energy production facilities is analyzed. Vertical air classification is one means of separating combustible organic matter from non-combustible inorganics typically found in municipal solid waste. Aerodynamically light organics ideally exit upward with air flow, while heavy inorganics ideally exit downward against the current. Problems develop in practice as organic matter is lost downward and inorganic matter exits upward to contaminate the fuel product. Utilizing a lab-scale vertical classifier, the overall efficiency and the sensitivity of the efficiency to air velocity is determined for several feed compositions. Particle-to-particle interaction is observed for a variety of feed compositions, and the effect this interaction has on the quality of the fuel product is documented.